<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>剩余宽高</title>
    <style>
        .box {
            width: 500px;
            height: 500px;
            border: 2px solid black;
            margin: 0 auto;
            display: flex;
            /*flex-direction: column;*/
        }

        .box div {
            width: 100px;
            height: 100px;
            border: 1px dashed red;
        }

        .div1,.div3 {
            flex: 1;
        }
        .div2 {
            flex: 1;/* 沾满剩余空间 */
        }
    </style>
</head>
<body>
<div class="box">
    <div class="div1"></div>
    <div class="div2"></div>
    <div class="div3"></div>
</div>
</body>
</html>